1. Field of Invention
The present invention relates generally to a steering system for a vehicle and more particularly to a vehicle steer-by-wire control system using a model-matching strategy.
2. Discussion of Related Art
In a steer-by-wire control system, the traditional steering column, steering shaft, and steering gear are replaced by an electronically controlled actuation system. The actuation system receives a steering command from a controller and accordingly turns the front road wheels using an electric motor. The vehicle driver provides the steering command through a steering wheel control system using a steering wheel or a joystick. As part of a steer-by-wire control system, a steering wheel control system with a force feedback motor actuator provides the direction reference signal for the road wheel actuation system and the familiar steering feel experienced in the conventional steering systems with a mechanical connection. Such a steer-by wire control system can improve a vehicle""s handling performance and stability by providing new features such as adjustable driver steering feel, variable steering effort, variable steering ratio, and active steering wheel return.
FIG. 1 schematically shows a typical steer-by-wire control system 100. As shown in FIG. 1, the steer-by-wire control system 100 includes a steering wheel unit 102, a road-wheel actuation unit 104 and a controller 106. The steering wheel unit 102 includes a steering wheel 108 that is actuated by the driver, a steering wheel angle sensor 110, a steering wheel actuator 112 with electric motor, a motor amplifier 114 and a steering column and connecting shaft 116. The road-wheel actuation unit 104 includes one or more road wheels 118, a road wheel actuator 120 with electric motor, a motor amplifier 122 and a road wheel angle sensor 124. Typically, there are two road wheels 118, one or two road wheel actuators 120 or one or two road wheel angle sensors 124.
The controller 106 controls the steering wheel unit 102 and the road-wheel actuation unit 104 via control algorithms that are implemented in the controller 106. In this system, the required steering functions are realized by using various control system design techniques. In current steer-by-wire control systems, driver""s steering feel is realized by controlling the reaction torque of a DC electric motor actuator 112 attached to the steering wheel 108. Typically, this reaction torque is a function of the steering wheel angle, vehicle speed, and road wheel torque (wheel""s self-aligning moment). It is a challenge for a control system designer to realize the required steering feel by controlling the reaction torque. In addition, in current steer-by-wire control systems, the steering feel and all other steering functions, such as steering wheel returnability, are coupled together. In other words, the controller parameters that affect the steering feel also influence the steering wheel returnability. This coupling between various controlled variables makes the design of a control system and tuning of the controller parameters very difficult and challenging.
A first aspect of the present invention regards a steer-by-wire control system that includes a road wheel actuation control system controlling road wheel angles to track a road wheel reference angle and a steering wheel control system producing a steering wheel angle to the road wheel actuation control system, steering feel for the vehicle driver and active steering wheel return. The steering wheel control system further including a steering wheel model matching controller that receives an input signal from the steering wheel control system and an input signal from the road wheel control system and generates a control output signal based on a model-matching control law, wherein the control output signal controls a reaction torque of the steering wheel control system to produce the steering feel for the driver and the return rate of a steering wheel or a joystick.
In a variation of the first aspect of the invention, the model matching controller has two degrees of freedom and receives a steering wheel angle signal and a road wheel torque signal.
A second aspect of the present invention regards a method of designing a steering wheel model matching controller using a model-matching control methodology. The design of the steering wheel model matching controller requires a reference steering dynamic model which provides a relationship between a steering wheel torque and a steering wheel angle. This reference steering model is representative of a relationship between a steering wheel reaction torque and a steering wheel angle which is applied as a typical evaluation for the steering feel. On the other hand, the steering wheel control system with the steering wheel model matching controller is designed as a closed-loop control system with the steering wheel angle as output and the steering wheel torque as input. With same input and output variables, the steering wheel control system matches the reference steering dynamic model using the different minimization criterions in the model-matching control design such that the steering wheel model matching controller is obtained by minimizing the error between the reference steering model and the steering wheel control system.
In a variation of the second aspect of the invention, the model matching controller with two degrees of freedom is determined using H∞ norm-based minimization criterion
Each aspect of the present invention provides the advantage of improving a driver""s steering feel, and the active steering wheel returnability.
Each aspect of the present invention provides the advantage of improving stability of a closed loop control system.
Each aspect of the present invention provides the advantage of improving the robustness of a steer-by-wire control system to disturbances and system parameter variations.
Each aspect of the present invention provides the advantage of adjusting parameters of a two-degree-of-freedom steering wheel model matching controller independently for improving the steering feel and steering wheel returnability.
Each aspect of the present invention provides the advantage of controlling the feel and rate of return of a steering wheel or joystick independently of one another.